© Semiconductor Components Industries, LLC, 2015
May, 2015 − Rev. P0 1 Publication Order Number:
EVBUM2299/D
Advance Information
NCV7691 8-Channel Rear Combo / Daylight Running Light Evaluation Board User'sManual
Introduction
The evaluation board demonstrates rear combination / daylight running light (RCL/DRL) solution with up to eight LED string. The board regulates constant current of 100 mA (68 to 150 mA) through each on−board LED string consisting of three red or two white LEDs in the wide range of the supply voltage. The current is reduced for the board temperature above 90°C. The board is designed in order to provide good cooling of the active components with no extra cost. Additionally, a heat sink may be mounted on the bottom side of the board to improve the thermal capability of the board. In case of LEDs short or opening, the current sources are disabled and the fault may be detected by the ECU diagnostics.
On−board LEDs may be easily replaced by external ones.
Evaluation Board Features
•
Wide Range of Supply Voltage: 7 to 40 V•
Eight LED Strings with On−board Red or White LEDs (RCL/DRL)•
Nominal LED Current 100 mA (150 mA max.)•
Over−temperature Current Fold−back Protection•
OPEN Jumper in each String Allowing LEDs Disconnection•
SHORT Jumper in each String for Short Emulation or Connection of the External LEDs•
Led Short−circuit and Open−load Emulation Jumpers•
Optional External PWM Input•
Optional Error Flagging through Indication LED and FLTS Output•
Positions for Optional EMC Capacitors•
One−side Components Assembly•
Passive Cooling (Optional Heat Sink may be Attached to the Bottom)Figure 1. Running NCV7691 Evaluation Board Picture
This document contains information on a new product. Specifications and information herein are subject to change without notice.
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EVAL BOARD USER’S MANUAL
Table 1. ABSOLUTE MAXIMUM RATINGS
Rating Value Unit
Supply Voltage (Vbat) −40 to +40 V
LED String Current (thermally limited) 150 mA
PWM, FLTS, Err Voltage (J1 connector) −0.3 to Vbat V
Error pin current (J1 connector, pin 3) 0 to 200 mA
Junction Temperature (NCV7691, BCP56) −40 to +150 °C
Junction Temperature (ASMT−QRBD−AEF0E, ASMT−QWBF−NKL0E) −40 to +125 °C
Ambient Temperature −40 to +105 °C
Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality should not be assumed, damage may occur and reliability may be affected.
Table 2. RECOMMENDED BOARD OPERATING CONDITIONS
Rating Value Unit
Supply Voltage (Vbat) 7 to 18 V
Rated LED string Current (VS < 18 V, Tboard < +90°C) 68 /100 /150 mA
Open−Load or Short−Circuit board consumption (VS = 14 V) max. 7 mA
Ambient Temperature (for rated LED current) −40 to +70 °C
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REAR COMBO / DAYLIGHT RUNNING LIGHT EVALUATION BOARD SCHEMATIC
Figure 2. NCV7691 RCL/DRL Evaluation Board Schematic
Table 3. INTERFACE FUNCTION DESCRIPTION
Connector Name Connector Type Description / Function
VBAT 2.1 mm DC supply Supply battery input
OPEN1−8 SMD Jumper Removing the jumper emulates open LED condition
SHORT1−8 SMD Jumper Shorting terminals by the jumper emulates short LED condition OR terminal for external LEDs
B Test Point BASE output of NCV7691 / NPN base
Col1 Test Point String 1 NPN collector
Col5 Test Point String 5 NPN collector
FB Test Point FB output of NCV7691
FB1 Test Point String 1 NPN emitter / feedback resistor FB5 Test Point String 5 NPN emitter / feedback resistor
J1 SMD Header Control/diagnostic functions:
Pin 1: GND
Pin 2: PWM input (10k pull−up to VS)
Pin 3: Error signal (active low, pull−up to VS through LED and 10k) Pin 4: FLTS pin of NCV7691
Pin 5: Alternative pull−up input for the error signal (if used, remove R9’)
GETTING STARTED The board is supplied through a standard 5.5 x 2.1 mm DC
connector. Nominal supply voltage ranges from 7 to 18 V.
Below 7 V, the current through the LEDs starts decreasing.
For battery voltage above 18 V, the LED current is reduced by the over−voltage fold back to reduce the power dissipation.
The thermal fold−back circuitry ensures the board temperature remains within the safe range (see following chapter for details).
Two jumpers in each string emulate LEDs short or opening. By default, OPEN jumper is closed while SHORT terminals are not shorted. Alternatively, external LEDs may
be connected through SHORT header while on−board LEDs are disconnected.
The LEDs current is defined by Rx1 resistors. By default, it is set to 100 mA, but may be changed using following formula:
•
With no thermal fold−back (NTC pin grounded):R1+FB Regulation Voltage
ILED [0.15
ILED
•
With thermal fold−back circuitry on NTC pin:R1+ VNTC
10@ILED[ 1.5
10@ILED+0.15 ILED
Figure 3. NCV7691 8−channel Evaluation Board Picture NCV7691
Thermal fold−back circuitry 7 to 18 V 2.1 mm DC supply
Positions for optional EMC resistors LED current programming resistors
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Modules Control Strategy
The board is designed to operate in standard “One Wire Driver Body ECU” and decentralized architecture.
In the first case, both control and diagnosis is done through one supply wire. The dimming might be provided through the PWM applied to the High−side SmartFET in the body ECU. At the same time, the SmartFET allows
diagnosis of the failure on the LED module (open load or short−circuit) (see Figure 3). In case of both LED short and disconnection, NCV7691 disables all the NPN bipolars.
In decentralized operation, extra wires are needed for the control and diagnosis (Figure 4). For this purpose, header J1 is available on the board for easy access to FLTS and PWM functions of NCV7691.
Figure 4. Centralized One Wire System Diagram ECU
NCV8460A
RCL / DRL
NCV7691
0.8 A
control diagnostics
7 mA max.
@ 14 V 7 mA max.
@ 14 V
open short
Normal operation LED string open LED string short
ECU
NCV8460A control
diagnostics
ECU
NCV8460A control
diagnostics
RCL / DRL
NCV7691
RCL / DRL
NCV7691
8x 100 mA
Figure 5. De−centralized System Diagram ECU
RCL / DRL
NCV7691 Control / diagnostics
ECU
RCL / DRL
NCV7691 Control / diagnostics
ECU
RCL / DRL
NCV7691 Control / diagnostics
error error
open short
Normal operation LED string open LED string short
Open LED String Diagnostics
NCV7691 includes Open Load Detection feature. When the LED is open, the control loop tries to increase the BASE current to reach 152 mV on the FB pin. As the BASE current is limited to typ. 25 mA, the FB voltage drops below the target level. When the FB voltage decreases below the Open Load Detection FB Threshold (typ. 75 mA), an open load is detected after a period defined by a capacitor connected to the FLTS pin. Figure 6 demonstrates an open load situation on two−string application.
Figure 6. Open Load Detection Principle
BCP56 BCP56
1R5 1R5
SC BASE FB GND NCV7691
63.6mV
37.2mV 90mV
60mA 24.8mA
25mA
~ 0.2mA ~ 24.8mA
V(FB) < 75 mV−>
−> FLTS current source active −>
−> LEDs off
In multi−string applications with high−beta transistors, the feedback voltage from individual strings is averaged, so one defective LED string does not always lead to the open load detection.
One of the ways to improve the open load detection capability is more precise external BASE current limitation.
An example of the circuit with one extra resistor and PNP bipolar is shown in Figure 6.
Figure 7. Improved Open Load Detection for 8 Strings
BCP56 BCP56
1R5 1R5
SC BASE FB GND
NCV7691 45mV 14mV
30mA 10.9mA
~ 10mA max.
~ 0.1mA ~ 9.3mA
V(FB) < 75 mV−>
−> FLTS current source active −>
−> LEDs off 47R
BC856
57.3mV
Short LED Diagnostics
NCV7691 contains a LED string short detection based on LED string voltage measurement (pin SC). In case the voltage difference between the VS and SC pins drops below typ. 2 V (Figure 7), the BASE is switched off and FLTS current source is active to flag an error. In multi−string applications, an OR−circuitry (e.g. diodes) has to be used to cover all the strings.
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Figure 8. Short LED Detection Principle
BCP56 BCP56
1R5 1R5
SC BASE FB GND NCV7691
VS
VS−0.5V V(SC) > (VS−2 V)−>
−> FLTS current source active
−> LEDs off
Thermal Considerations of the Board
As the board dissipation is typ. 11 W at 13.8 V battery supply and 100 mA LED current, the thermal aspects should be taken into account for the board design. The main limitations are LED lifetime vs. operating temperature and NPN bipolar maximum junction temperature.
The board was designed to operate at full current with board temperature of up to 90°C on standard 70 mm FR4 PCB substrate and passive cooling without any extra thermal improvements. In case higher power dissipation is foreseen, a cooling metal profile may be mounted to the bottom side of the board.
Thermal Fold−back
To the protect power dissipating devices on the board (LEDs, NPN bipolar transistor), a circuit reducing LED current at high board temperatures is included (Figure 8).
Figure 9. Thermal Fold−back Circuitry
R4 100k
R6 560k
R5 51k 4V7
PTC R3
10k
Temperature dependent
element Voltage reference
T < Tsense: 470 Ω T = Tsense + 15°C: >40 kΩ
to NTC pin VS
~4.3 V
The circuit consists of a voltage reference (ZD1 supplied via R3) and temperature dependent resistor divider. A thermistor with positive temperature dependency (PTC) is
used for temperature sensing. For temperatures below the PTC sensing temperature, the thermistor has low resistance (typ. 470 W), so the circuitry output voltage is given by R4 and R5 while for high temperatures the reference voltage drops rapidly (Figure 9). R6 ensures the voltage on NTC input does not fall below the NTC Detection Level (max.
300 mV) at high temperature, which would result into a switch−over to the internal voltage reference of NCV7691.
Figure 10. Thermal Fold−back Circuitry at Low/High Temperature
R4 100 k
R6 560 k
R5 51k
PTC ~ 470 R to NTC pin 4.3 V
1.45 V
R4 100k
R6 560k
R5 51k
PTC³ R
to NTC pin 4.3 V
0.31 V
V(NTC) has to be > 0.3 V T < Tsense T >> Tsense
The optimal Zener voltage in terms of the temperature independency is usually between 4 and 5 V. At the same time, it should be as low as possible to allow a low−battery operation. 4V7 Zener diode is recommended as a good tradeoff. Because of lower bias current (< 1 mA) and impedance of the Zener, the reference voltage is lower than the nominal one (∼4.3 V for 4V7 Zener) (Figure 10).
Figure 11. NTC Voltage vs. Supply Voltage (Tpcb = 255C)
The sensing temperature (PTC parameter) has to be chosen carefully with respect to the heat distribution over the board and thermal properties of the power dissipating components. For PTC type B59641A95A62, the current fold−back reduces the reference voltage on the NTC pin and
thus LED current above the board temperature of ∼90°C (Figure 11).
Figure 12. NTC Voltage vs. Board Temperature (VS = 12 V)
If the thermal fold−back feature is not required, components R3, R4, R6, ZD1 and PTC do not need to be assembled and NTC pin should be tied to GND (directly or via R5).
EMC recommendations
If extensive EMC immunity level is required, Cx1, C3 and C4 capacitors can improve the EMC performance. Using C3 is usually sufficient against disturbances from the supply line. Capacitors Cx1 and C4 may further improve the performance esp. in the setups with external (off−board) LEDs.
PCB DRAWINGS Assembly Drawings
Figure 13. NCV7691 RCL EVB PCB Top Assembly Drawing (Red LEDs) Option A
Figure 14. NCV7691 DRL EVB PCB Top Assembly Drawing (White LEDs) Option B
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Composite Drawings
Figure 15. NCV7691 RCL/DRL EVB PCB Top Composite Drawing
Figure 16. NCV7691 RCL/DRL EVB PCB Bottom Composite Drawing
REFERENCES [1] ON Semiconductor, NCV7691 Product Datasheet
Rev. 2, January 2015
[2] EPCOS, PTC thermistors as limit temperature sensors, Series: B59421, B59641, B59721, March 2014
[3] ON Semiconductor, BCP56 NPN Silicon Epitaxial Transistor, Rev. 10, March, 2014.
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